2. Set a medium bowl on a dampened kitchen towel on the counter to keep it steady.

3. Halve the hard-boiled egg and separate the white from the yolk. Push the yolk through a sieve into the bowl (alternatively, mash it to a very fine aste with a fork). Mix in the mustard and salt until completely smooth.

4. Measure both oils into a measuring cup with a pouring spout. Pour it into the bowl a few drops at a time
at first, whisking constantly to allow the oil to form an emulsion with the other ingredients. Whisk with your
dominant hand and pour the oil with the other; the bowl should not move, thanks to the dampened towel.

5. When the mixture becomes creamy, you can start pouring the oil in a very thin drizzle, whisking all the while. When all the oil has been added, the mayonnaise should form peaks. If the sauce seems stiff, whisk in the vinegar to loosen.